Determination of benzene at low parts per trillion levels in seawater

This paper reports a sampling and analysis method, incorporating the technique of dynamic headspace sampling, which has been optimized for the rapid and precise determination of low ppt levels of benzene in seawater. One-liter, standard-taper glass reagent bottles are used as the combined sampling container and purge device reservoir. For sampling, the bottle is equipped with a teflon stopcock gas adapter, evacuated, connected via teflon tubing to a piece of glass tubing sealed on one end, and placed in a bucket attached to the ship's hydrowire. Filling is initiated at the desired depth by breaking the sealed glass tube with a messenger. For purging, the gas adapter is replaced with a gas-dispersion tube, a Tenax trap is attached, and the stirred, heated sample is purged with helium. The Tenax traps are designed to fit into the unmodified injection port of the gas chromatograph for thermal desorption. They consist of 6-mm o.d. by 4-mm i.d. glass tubes packed with a 5-cm length of Tenax GC. Sample temperature, purge rate, and purge time are all optimized for maximum benzene recovery, which averages 95%. The chromatographic column used is 0.3% Carbowax 20M on Carbopack C, 3.2 mm × 1.3 m stainless steel, programmed from 0°C to 180°C. Overall recovery precision is 14% RSD at a level of 15 ppt.